Kampuchea under Pol Pot - note Kampuchea was a member of the UN for 15 years AFTER this.

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Estimates of the total number of deaths resulting from Khmer Rouge policies, including disease and starvation, range from 1 to 3 million out of a population of around 8.4 million. François Ponchaud suggested 2.3 million, R.J. Rummel 2.4 million (counting democide in the civil wars), the Yale Cambodian Genocide Project 1.7 million, and Amnesty International 1.4 million. Demographer Marek Sliwinski concluded that at least 1.8 million were killed from 1975–9 on the basis of the total population decline, compared to roughly 40,000 killed by the U.S. bombing.[29] Researcher Craig Etcheson of the Documentation Center of Cambodia suggests that the death toll was between 2 and 2.5 million, with a "most likely" figure of 2.2 million. After five years of researching some 20,000 grave sites, he concludes that, "these mass graves contain the remains of 1,386,734 victims of execution."[30][31] Execution is believed to have accounted for about 50 percent of the death toll.

In the second half of the twentieth century, Americans were taught to see both Nazi Germany and the Soviet Union as the greatest of evils. Hitler was worse, because his regime propagated the unprecedented horror of the Holocaust, the attempt to eradicate an entire people on racial grounds. Yet Stalin was also worse, because his regime killed far, far more people, tens of millions it was often claimed, in the endless wastes of the Gulag. For decades, and even today, this confidence about the difference between the two regimes—quality versus quantity—has set the ground rules for the politics of memory. Even historians of the Holocaust generally take for granted that Stalin killed more people than Hitler, thus placing themselves under greater pressure to stress the special character of the Holocaust, since this is what made the Nazi regime worse than the Stalinist one.

Over the course of approximately 100 days (from the assassination of Juvénal Habyarimana and Cyprien Ntaryamira on April 6 through mid-July) over 500,000 people were killed, according to a Human Rights Watch estimate.[1] Estimates of the death toll have ranged from 500,000–1,000,000,

Holocaust was worse. The intent of the holocaust was the systematic elimination of an entire group of people. The intent of slavery was the importation of people for the purpose of providing forced labor and was not specifically genocidal. While Jews were used as labor that was as a matter of convenience and not the primary purpose of their captivity.

In terms of dollars and cents, the slave had a greater monetary value than the Jews in concentration camps. Importing and owning slaves was expensive and it would not have been in the best interest of the slave owner to kill a slave unless absolutely necessary. There are a pretty good counter arguments to the above.